Petrology and organic geochemistry of the Baishaping and Damaidi Devonian cutinitic liptobioliths, west of the Kangdian Uplift, China

Devonian cutinitic liptobiolith is a special type of coal formed during the early evolutionary stage of land plants. The Baishaping and Damaidi Devonian coals are notable for their distinct papery, sheet-like texture, and unusually high cutinite content (>50%), belonging to typical cutinitic liptobiolith. They are bituminous coals and have a suppressed vitrinite reflectance. Compared with typical type III kerogen, these liptobioliths are characterized by extremely high yields of volatile matter and high hydrogen contents owing to their enhanced cutinite contents. As indicated by pyrolysis and elemental analysis results, the Devonian cutinitic liptobioliths are dominated by type I-II kerogens and have an excellent potential to generate liquid hydrocarbons. The Baishaping cutinitic liptobioliths are mainly composed of ribbon-like thick-walled cutinites, with small amounts of thin-walled type. Vitrinite is only present in trace amounts and is dominated by telinite. Meanwhile, the Damaidi Devonian coals are primarily composed of ribbon-like thin-walled cutinites, followed by collotelinites which are usually sandwiched by cutinites. The different petrological characteristics of the Baishaping and Damaidi liptobioliths might indicate a structural variation in the coal-forming plant cuticles and two distinct coal-forming conditions. The molecular biomarkers in the Devonian coals are mainly composed of C18–C20 tetracyclic diterpenoids, which are assumed to be the dominant soluble constituent of the Devonian land-plant cuticles and might have played an important role in early land plant radiation. Other molecules include aromatic tetracyclic diterpenoids, 3–5 ring polycyclic aromatic hydrocarbons and their methylated counterparts, and C16- and C18-monocarboxylic acids. n-Alkanes are present only in small amounts. The molecular composition of the cutinite-rich coals is unexpected, because cuticular waxes are one of the most important sources of n-alkanes. In general, the petrology and geochemistry of the Devonian cutinitic liptobiolith indicate a distinct structure and composition for the early land-plant cuticles, which is significantly different from that of the plant cuticles since Carboniferous.